Method of making humidity sensor

ABSTRACT

The operation of relative humidity sensors made from cobalt oxide on a non-conductive ceramic substrate is improved by heating the sensor for a short period to a high temperature in a reducing atmosphere. This reduces the specific resistance of the device. With a lower resistivity, it is possible to reduce the size of the sensor to the point where it can be included along with a semiconductive device in standard hermetic packages. This makes possible the continuous, on-line monitoring of hermeticity for the life of the circuit without the necessity fo applying a load to the circuit.

United States Patent [191 Frazee et al.

1 1 June 24, 1975 METHOD OF MAKING HUMIDITY SENSOR [75] Inventors:Lawrence E. Frazee, Huntington,

N.Y.; Anthony V. Fraioli, Essexfells, NJ.

[73] Assignee: Plessey Incorporated, Melville, NY.

[22] Filed: Feb. 19, 1974 [21] Appl. No.: 443,436

[52] US. Cl. 29/621; 29/620; 252/518; 338/35 [51] Int. Cl. H0lc 1/14;H010 17/00 [58] Field of Search 29/620, 621, 610; 73/335, 73/3365;252/518; 338/35; 117/201; 148/13,

[56] References Cited UNITED STATES PATENTS 3,345,596 10/1967 Delaney etal 338/35 3,369,880 2/1968 Mochel 338/35 X 3,607,386 9/1971 Galla 29/620X 3,703,697 ll/l972 Nicholas 338/35 FOREIGN PATENTS OR APPLICATIONS795,031 5/1958 United Kingdom 338/35 Primary Examiner-Richard J. HerbstAssistant Examiner-Victor A. DiPalma Attorney, Agent, or Firm-James J.Burke, 11

[5 7] ABSTRACT The operation of relative humidity sensors made fromcobalt oxide on a non-conductive ceramic substrate is improved byheating the sensor for a short period to a high temperature in areducing atmosphere. This reduces the specific resistance of the device.With a lower resistivity, it is possible to reduce the size of thesensor to the point where it can be included along with a semiconductivedevice in standard hermetic packages. This makes possible thecontinuous, on-line monitoring of hermeticity for the life of thecircuit without the necessity fo applying a load to the circuit.

5 Claims, 3 Drawing Figures METHOD OF MAKING HUMIDITY SENSOR BACKGROUNDOF THE INVENTION The present invention relates in general to relativehumidity sensors and, more particularly, the invention relates tosensors made from cobalt oxide as the active element. Humidity sensorsof this type are also referred to as hygrometers or humistors, as it istheir electrical resistivity which changes with humidity.

Procedures for manufacturing cobalt oxide humidity sensors orhygrometers are well known. The starting material is a cobalt oxidepowder. As pure C powder is very expensive, the starting material isgenerally a mixture of C00 with some C0 0 but the latter compounddissociates at about 900C. so the completed sensor will be essentiallyCoO, the cobaltous oxide. This compound is stable up to its meltingpoint, which is above l800C.

The finely divided powder, preferably minus 325 mesh, is mixed with aninert liquid vehicle and viscosifiers to form a screen-printable paste.A thin layer of the paste is then screen-printed in a desired patternonto a dielectric, high-temperature resistant substrate, typically ahigh-alumina ceramic. The screened pattern is then dried and fired inair at a temperature in the range of 1350C. to 1550C. Electrodes can bepreformed on the substrate, co-fired with the paste, or applied in asubsequent operation. The latter is the more common approach, as it isgenerally desired to have the electrodes in a rather elaborate,interdigitated pattern on the top surface. Conductive inks or pastes(platinumgold, palladium-gold, etc.) are used in the conventionalmanner.

Before such a sensor can be put to use, it must be accurately calibratedto determine the change in electrical resistance with relative humidity.

The very high specific resistivity of cobalt oxide,

which is ohms per square or higher, requires that humidity sensors madetherefrom be relatively large in order to produce suitable suitalbeoutput. Typically, prior art humidity sensors of this type aremanufactured on one inch square substrates. This is of course much toolarge to be included in integrated circuit packages.

The desirability of having a humidity sensor within a hermeticallysealed semiconductor package is manifest. Essentially all semiconductivedevices are humidity-sensitive to a greater or lesser degree. As aresult, specifications on hermeticity for packages are most stringent.But, while technology for producing hermetic packages is well developedand tests therefor standarized, the fact that a package is hermeticinitially says nothing about whether it will remain so after months oryears of service, often under severe conditions of shock and vibration.Further, the failure of a seal in service can now be detected only by amalfunction or failure of the circuit.

PRIOR ART The production of cobalt oxide hygrometers is disclosed byDelaney et al in u.S. Pat. No. 3,345,596. The two U.S. Pat Nos. ofNicholas, 3,703,697 and 3,715,702, disclose the use of humectants toincrease water absorption on hygrometer surfaces of this same type. Inone instance the oxide is converted in part to the oxychloride, and inthe other, a coating of lithium chloride or polyethylene glycol isprovided. Blythe et al, U.S. Pat. No. 3,105,214, disclose the use ofavaporpermeable ion-selective membrane on the sensor surface. This willswell up in a humid environment and transport water but not ions to thesensor surface.

OBJECT OF THE INVENTION A general object of the present invention is toprovide improved humidity sensors of the cobalt oxide type.

Another object of the present invention is to provide a cobalt oxidehumidity sensor which is smaller than prior art sensors.

A further object of the present invention is to provide a cobalt oxidehumidity sensor within a hermetically sealed semiconductor package.

A still further object of the present invention is to provide a cobaltoxide humidity sensor having a lower resistivity than prior art sensorsof the same general type.

Yet another object of the present invention is to provide a cobalt oxidehumidity sensor very sensitive in the low-ppm range.

A still further object of the present invention is to provide asemiconductor package with a built-in or integral humidity sensor.

Various other objects and advantages of the invention will become clearfrom the following description of embodiments thereof, and the novelfeatures will be particularly pointed out in connection with theappended claims.

THE DRAWINGS Reference will hereinafter be made to the accompanyingdrawings wherein:

FIG. 1 is a plan view of a typical humidity sensor;

FIG. 2 is a perspective view of a portion of an integrated circuitpackage including an integral humidity sensor in accordance with thisinvention; and

FIG. 3 is an enlarged view of a portion of the sensor of FIG. 1,illustrating resistivity calculations.

DESCRIPTION OF EMBODIMENTS The present invention comprises, in essence,lowering the specific-resistivity of the cobalt oxide film by heatingthe previously fired film to a high temperature in a reducing atmospherefor a brief period. The atmosphere is preferably hydrogen. Thistreatment has been observed to reduce the resistivity of the film by lto 1.25 orders of magnitude. Thus, an air-fired CoO film with aresistivity of 3.76 X 10 ohms/square has a resistivity of 6.3 X 10 ohmsper square after firing in hydrogen at l500C. for 15 minutes.

The reasons why this treatment reduces resistivity are unclear, just asthe mechanism of conduction in cobalt oxide is unclear. It is possiblethat the reducing gas attacks the COC crystallite boundariespreferentially and exposes more surface with a higher surface energy. Itis not known whether CoO conducts with surface electrons or holes in thebulk material; it is felt to be possible that the hydrogen firing couldcontribute to bulk conductivity because of the higher surface energyconditions.

It is believed that the prior-art (Delaney et al) teaching of firing thecobalt oxide for a brief period only and at a high temperature (1500C.)is designed to provide a large number of very small CoO crystals andprevent crystal growth. The hydrogen treatment of the present 3invention may significantly increase the populationdensity of exposedcrystal edges and corners. Because of the interrupted latticeperiodicity at these locations, and resultant dangling or broken bonds,these sites should be preferred locations for gas adsorptiondesorptionphenomena involved in the relative humidity equilibration process.

The hydrogen firing process has been optically observed to produce etchpits on the C surface which are quasi-hexagonal in plan view and conicalin crosssection, with terraced walls. Etch lines about larger,pyramidal-shaped crystals parallel to a base plane have also beobserved. It is believed that such exposed crystal surfaces shouldenhance gas-surface interaction phenomena.

The lowering of the C00 resistivity is desirable for several reasons,the most important of which is that it permits a substantial reductionin the size of the humistor. Thus, heretofore a typical humistor had tobe mounted on a 1 inch square substrate, to allow for device andconductive geometry that would give a suitable output. In accordancewith the present invention, humistors of the same resistance areproduced on 0.20 by 0.20 by 0.040 inch substrates with the same (1-5volt' outputs and improved linearity of response.

This permits humsitors of the present invention to be used for the firsttime as hermeticity detectors within semiconductive device packages. Asnoted hereinabove, stringent hermeticity requirements are imposed inboth military and commercial electronics specfications, because humidityhas a disastrous effect on the operation of both active and passivemicrocircuit elements. Heretofore, it has not been possible to detecthermeticity failures, i.e., leaks, except through circuit mal-functions.With the miniaturized humidity sensors produced in accordance with thepresent invention, it becomes possible to include the device on thesubstrate within the can or package. In operation with a nominal bias,it will be essentially non-conductive. If the package fails and even afew ppm of moisture are encountered, the sensor will be renderedsufficiently conductive to trigger an alarm. Thus, with the presentinvention it is possible to continuously monitor hermeticity withoutwaiting for circuit damage, mal-function or failure, and hermeticitymonitoring is carried out without loading the main circuit. Moreparticularly, humistors of the present invention may have a totalwet-todry resistance change of as much as six or seven orders ofmagnitude. This very great change is believed to account for thesubstantial sensitivity in the very low ppm (high resistivity) range.

Humistors made in accordance with the present invention resemble priorart devices except insofar as the hydrogen firing tends to change thenormally rather glossy CoO surface to a more matte-like appearance.

As shown in FIG. 1, a ceramic substrate has a layer of cobalt oxide 12screened and fired over its entire surface (or less than the entiresurface, as desired). First and second electrode patterns 14, 16 whichterminate in bonding pads 18, 20 are then screened and fired. Afterfiring in a reducing atmosphere (which may occur before or afterapplication of electrodes) the device is complete.

FIG. 2 illustrates a humidity sensor of the present inventionincorporated into a standard dual in-line 14- lead package. Thesubstrate 22 first has a cobalt oxide area 24 screened and firedthereon. Thereafter, the integrated circuit leads 26, the humistorelectrodes 28, and leads therefore 30 are all screened and firedsimultaneously. The entire unit is then fired in hydrogen,

EXAMPLE In humistors (or resistors) of the general type described,calculation of resistivity in ohms/squre is more complex than for simplethick film geometries, and depends on the space between respectiveinterdigitated electrodes and their length. With reference to FIG. 3,the number of squares is calculated according to the following formula:

No. of uares X Sq W In Equation 1, N is the total number of conductorlines, so NJ is the number of resistor lines. The factor L/\ V ismultiplied by the reciprocal of N-l because, while L is constantthroughout the pattern, W is lengthened by each pair of conductors, andit is this which must be multiplied by N-l. As will be apparent from thedata hereinbelow, this produces resistors with very small numbers ofsquares.

Cobalt oxide paste was screened and fired. onto one inch substrates in a950 mil square pattern and fired. One group of substrates was fired inhydrogen and all substrates had identical conductives applied, wherein(all dimension being in mils). In accordance with equation 1, the numberof squares was 7.5 X 10. At percent relative humidity, the resistorsthat were not hydrogen fired had resistances of 3.2 X 10 0 and thosethat were so treated measured 1.9 X 10 0. In terms of ohms/square, thesefigures convert to 4.26 X 10 and 2.53 X 10 respectively.

The same paste was screened in 200 mil square patterns and, again onegroup was fired in hydrogen and another was not. Conductives wereapplied in a pattern where The number of squares in this pattern is1.19047 X 10-. During processing, film thickness, firing parameters etc.were all closely controlled so that the films were comparable except forsize and conductive geometry. On these units, at 100 percent R.I-l.,resistance of the non-hydrogen fired units was 5.07 X 10 and those thatwere so treated measured 3.01 X 10 0. Resistance of 200 mil squarehumistors treated in accordance with the present invention is thus seento be substantially the same as 950 mil square units that were not sotreated (i.e., 3.2 X IO Q).

Humistors are calibrated by plotting log R vs. relative humidity overthe entire humidity range. A problem with known humistors has been anasymptotic resistance change (or other anomalides) as R.H. approaches 0percent. An advantage of humistors made in accordance with the presentinvention is a substantially linear response over the entire RH. range.This greatly simplifies required circuitry and makes the devices usefulas hermeticity detectors, where R.I-I. must be measured inparts-per-million. The reason why the reducing-gas treatment bringsabout this desirable improvement is not understood.

It will be appreciated by those skilled in the art that FIG. 2 isillustrative only and is not to be construed in a limiting sense, sincemany variations are possible. Thus, a discrete humistor as shown in FIG.1 could be incorporated into a semiconductor package rather than havingthe package manufactured with an integral device. The humistor could bewired inot the main circuit if that condition could be tolerated.Different package designs offer varying opportunities for placement ofthe sensor. In packages having glass-sealed ceramic lids for example, itwould be possible to have the sensor printed on the under-side of thelid. Discrete or integral sensors could be incorporated into headercans. In laminated packages having leads on a raised step sectionsurrounding the device in a central cavity, the sensor could be placedon either level.

It will be further appreciated that the small size of humistors of thepresent invention permits their incorporation in a variety of otherdevices where hermeticity is important. Image intensifier tubes or otherelectronic tubes are one example. Optical systems where hermeticity isrequired to avoid condensation of optical elements in another example.

Various other changes in the details, steps, materials and arrangementsof parts, which have been herein described and illustrated in order toexplain the nature of the invention, may be made by those skilled in theart within the principle and scope of the invention as defined in theappended claims.

What is claimed is:

1. The method of forming a humidity sensor comprising:

mixing cobalt oxide powder with an inert vehicle to form a paste;applying a layer of said paste onto a major surface of ahigh-temperature resistant dielectric substrate;

firing said paste in an oxidizing atmosphere at a temperature of aboutl350C. to 1550C. to form a hygroscopic element; and

firing said hygroscopic element at a temperature of about l500C. for afew minutes in a reducing atmosphere, whereby the resistivity of saidelement is reduced.

2. The method as claimed in claim 1, and additionally comprisingapplying a pair of electrodes to said element.

3. The method as claimed in claim 1, wherein said reducing atmospherecomprises hydrogen.

4. In the method of producing a humidity sensor by firing a layer of acobalt oxide paste onto a hightemperature resistant, dielectricsubstrate to produce a hygroscopic element, the improvement comprisingfiring said element a second time for a short period at a temperature ofabout 1500C. in a reducing atmosphere, whereby the resistivity of saidelement is lowered.

5. The method as claimed in claim 4, wherein said reducing atmosphere ishydrogen.

UNITED STATES PATENT OFFICE CE TIFICATE F CC Patent No. 3 Dated 24 June1975 (s) E et a].

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 40: 'suitable suitalbe" should be --a suitable-; line 61:"u.S." should be --U,S.--

Column 3, line 13: 'be" should be been-= II 22" "2 Column 5, line 5: 10should be -lO lines 6 and 14 should not be indented; line 18:"anomalides" should be --anomalies--; line 33: "inot" should be--=into-- Signed and 5-- this [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissinner nflarenlsand Trademarks ninth Day of September 1975

1. The method of forming a humidity sensor comprising: mixing cobaltoxide powder with an inert vehicle to form a paste; applying a layer ofsaid paste onto a major surface of a high-temperature resistantdielectric substrate; firing said paste in an oxidizing atmosphere at atemperature of about 1350*C. to 1550*C. to form a hygroscopic element;and firing said hygroscopic element at a temperature of about 1500*C.for a few minutes in a reducing atmosphere, whereby the resistivity ofsaid element is reduced.
 2. The method as claimed in claim 1, andadditionally comprising applying a pair of electrodes to said element.3. The method as claimed in claim 1, wherein said reducing atmospherecomprises hydrogen.
 4. IN THE METHOD OF PRODUCING A HUMIDITY SENSOR BYFIRING A LAYER OF A COBALT OXIDE PASTE ONTO A HIGH-TEMPERATURERESISTANT, DIELECTRIC SUBSTRATE TO PRODUCE A HYGROSCOPIC ELEMENT; THEIMPROVEMENT COMPRISING FIRING SAID ELEMMENT A SECOND TIME FOR A SHORTPERIOD AT A TEMPERATURE OF ABOUT 1500*C. IN A REDUCING ATMOSPHERE,WHEREBY THE RESISTIVITY OF SAID ELENENT IS LOWERED.
 5. The method asclaimed in claim 4, wherein said reducing atmosphere is hydrogen.